New insights into the subglacial and periglacial hydrology of Vatnajokull, Iceland, from a distributed physical model

We apply a time-dependent distributed glaciohydraulic model to Vatnajokull ice cap, Iceland, aiming to determine the large-scale subglacial drainage structure, the importance of basally derived meltwater, the influence of a permeable glacier bed and Vatnajokull's discharge contribution to major rivers in Iceland. The model comprises two coupled layers that represent the subglacial horizon perched on a subsurface aquifer in the western sector and bedrock in the eastern sector. To initialize and drive the simulations, we use digital elevation models of the ice surface and bed, the 1999/2000 measured mass balance and an estimate of subglacial geothermal heat fluxes. The modelled subglacial flow field differs substantially from that derived by hydraulic-potential calculations, and the corresponding distribution of basal effective pressure shows a strong correlation between low effective pressure and surge-prone areas in northeastern and southern sectors of Vatnajokull. Simulations suggest that geothermally derived basal melt may account for up to similar to5% of the annual glacial discharge, and buried aquifers may evacuate up to similar to30% of subglacial water. Time-dependent tests yield estimates of the glacial discharge component in various outlet rivers and suggest a possible seasonal migration of subglacial hydraulic divides. This study of present-day Vatnajokull hydrology forms the starting point for investigations of its future evolution.